Australian police have ended ended their investigation of a lawmaker in Prime Minister Julia Gillard's Labor Party that could have erased her one-seat majority in parliament.

Craig Thomson was accused of using a credit card issued by the Health Services Union to pay for prostitutes while he was union's national secretary, before he entered parliament in 2007.

The union turned over Thomson's credit card information to police in New South Wales state, who opened a fraud investigation. But officials said Thursday there is no basis for a criminal probe.

If Thomson had been tried and convicted, it would have forced a special election that likely would have gone against Ms. Gillard and her Labor Party, whose approval ratings are at an all-time low.

Deputy Prime Minister Wayne Swan said the opposition Liberal party used the probe to engage in mud-slinging against Labor.